diff --git a/frontend/src/lib/components/ErrorOrRecoveryHandler.svelte b/frontend/src/lib/components/ErrorOrRecoveryHandler.svelte
index aab564a4cf..c624b5a148 100644
--- a/frontend/src/lib/components/ErrorOrRecoveryHandler.svelte
+++ b/frontend/src/lib/components/ErrorOrRecoveryHandler.svelte
@@ -69,6 +69,8 @@
 	} from 'lucide-svelte'
 	import { hubPaths } from '$lib/hub'
 	import { isCloudHosted } from '$lib/cloud'
+	import { untrack } from 'svelte'
+	import { syncSlackBotArg } from './handlerSlackArg'
 	import SmtpConfigurationStatus from './common/smtp/SmtpConfigurationStatus.svelte'
 	import { SettingService } from '$lib/gen'
 	import { isSmtpSettingsValid } from './instanceSettings/SmtpSettings.svelte'
@@ -316,12 +318,13 @@
 		loadSmtpConfiguration()
 	})
 
+	let builtinArgs: Record<string, any> | undefined
 	$effect(() => {
-		if (handlerSelected === 'slack' && isSlackHandler(handlerPath)) {
-			handlerExtraArgs['slack'] = '$res:f/slack_bot/bot_token'
-		} else {
-			handlerExtraArgs['slack'] = undefined
-		}
+		const isSlackBuiltin = handlerSelected === 'slack' && isSlackHandler(handlerPath)
+		const args = handlerExtraArgs
+		// A replaced args object (Discard, reload) holds the stored config, not what the built-in set.
+		untrack(() => syncSlackBotArg(args, isSlackBuiltin, args === builtinArgs))
+		builtinArgs = isSlackBuiltin ? args : undefined
 	})
 
 	let lastHandlerSelected: ErrorHandler | undefined = $state(undefined)
diff --git a/frontend/src/lib/components/handlerSlackArg.test.ts b/frontend/src/lib/components/handlerSlackArg.test.ts
new file mode 100644
index 0000000000..b087a55f26
--- /dev/null
+++ b/frontend/src/lib/components/handlerSlackArg.test.ts
@@ -0,0 +1,21 @@
+import { describe, expect, it } from 'vitest'
+import { syncSlackBotArg } from './handlerSlackArg'
+
+const AUTO = '$res:f/slack_bot/bot_token'
+
+describe('syncSlackBotArg', () => {
+	it("leaves a custom handler's slack value alone", () => {
+		const args = { slack: '$res:u/me/my_slack' }
+		syncSlackBotArg(args, false, false)
+		syncSlackBotArg(args, false, true)
+		expect(args).toEqual({ slack: '$res:u/me/my_slack' })
+	})
+
+	it('sets the bot token for the built-in and drops it when leaving', () => {
+		const args: Record<string, any> = {}
+		syncSlackBotArg(args, true, false)
+		expect(args.slack).toBe(AUTO)
+		syncSlackBotArg(args, false, true)
+		expect(args).toStrictEqual({})
+	})
+})
diff --git a/frontend/src/lib/components/handlerSlackArg.ts b/frontend/src/lib/components/handlerSlackArg.ts
new file mode 100644
index 0000000000..6b8e4d6f5c
--- /dev/null
+++ b/frontend/src/lib/components/handlerSlackArg.ts
@@ -0,0 +1,17 @@
+import { WORKSPACE_SLACK_BOT_TOKEN_PATH } from '$lib/consts'
+
+const AUTO_SLACK_ARG = '$res:' + WORKSPACE_SLACK_BOT_TOKEN_PATH
+
+// `slack` belongs to the Slack built-in only while it holds the value the picker set; any other
+// value is a custom handler's own argument and must survive handler switches and reloads.
+export function syncSlackBotArg(
+	args: Record<string, any>,
+	isSlackBuiltin: boolean,
+	wasSlackBuiltin: boolean
+) {
+	if (isSlackBuiltin) {
+		args.slack = AUTO_SLACK_ARG
+	} else if (wasSlackBuiltin && args.slack === AUTO_SLACK_ARG) {
+		delete args.slack
+	}
+}
diff --git a/frontend/src/routes/(root)/(logged)/workspace_settings/+page.svelte b/frontend/src/routes/(root)/(logged)/workspace_settings/+page.svelte
index 7228cf9f48..58f9d5d897 100644
--- a/frontend/src/routes/(root)/(logged)/workspace_settings/+page.svelte
+++ b/frontend/src/routes/(root)/(logged)/workspace_settings/+page.svelte
@@ -1133,10 +1133,14 @@
 	}
 
 	// Strip keys from extraArgs that are auto-managed by child components:
-	// - 'slack': computed by ErrorOrRecoveryHandler's $effect based on handler type
+	// - 'slack' while the Slack built-in is selected: set by ErrorOrRecoveryHandler's $effect
 	// - 'channel_name': display metadata stripped by SchemaForm's removeExtraKey()
-	function normalizeHandlerExtraArgs(args: Record<string, any>): Record<string, any> {
-		const { slack: _, channel_name: __, ...rest } = args
+	function normalizeHandlerExtraArgs(
+		args: Record<string, any>,
+		handlerSelected: ErrorHandler
+	): Record<string, any> {
+		const { channel_name: _, ...rest } = args
+		if (handlerSelected === 'slack') delete rest.slack
 		return rest
 	}
 
@@ -1146,7 +1150,10 @@
 			errorHandlerSelected: initialErrorHandlerSelected,
 			errorHandlerScriptPath: initialErrorHandlerScriptPath,
 			errorHandlerItemKind: initialErrorHandlerItemKind,
-			errorHandlerExtraArgs: normalizeHandlerExtraArgs(initialErrorHandlerExtraArgs),
+			errorHandlerExtraArgs: normalizeHandlerExtraArgs(
+				initialErrorHandlerExtraArgs,
+				initialErrorHandlerSelected
+			),
 			errorHandlerMutedOnCancel: initialErrorHandlerMutedOnCancel,
 			errorHandlerMutedOnUserPath: initialErrorHandlerMutedOnUserPath
 		}
@@ -1155,7 +1162,7 @@
 			errorHandlerSelected: errorHandlerSelected,
 			errorHandlerScriptPath: errorHandlerScriptPath,
 			errorHandlerItemKind: errorHandlerItemKind,
-			errorHandlerExtraArgs: normalizeHandlerExtraArgs(errorHandlerExtraArgs),
+			errorHandlerExtraArgs: normalizeHandlerExtraArgs(errorHandlerExtraArgs, errorHandlerSelected),
 			errorHandlerMutedOnCancel: errorHandlerMutedOnCancel,
 			errorHandlerMutedOnUserPath: errorHandlerMutedOnUserPath
 		}